Typically, a nuclear reactor has a vessel containing a core made up of a plurality of fuel assemblies. The lower regions of the fuel assemblies include narrow annuli within a fuel assembly flow distribution plate and a nozzle block. In-core instrumentation for monitoring reactor operations is supplied through the vessel and distribution plate into the core and reside in one or more thimble guide tubes which provide a guide path for the insertion of the instrumentation into the core. The instruments may be movable within the thimble guide tubes to monitor operation at various axial locations in the fuel assemblies. Typically, a small amount of the reactor coolant water circulates through the instrument thimble tube as well as about the fuel rod assembly.
It has been found that the coolant water flow through the narrow annuli between a flow distribution plate and the core lower support plate and around the thimble guide tubes, at the entry point of the instrumentation into the thimble guide tube, causes vibration and interaction between the thimble tube and adjacent structures which cause fretting wear and damage to the instrumentation, which may result in premature failure of the instrumentation.